Many mobile communication systems employ multiple coverage areas to accommodate necessary mobile communication over a defined region. Of such systems, simulcast communication systems and cellular communication systems are the most common types which provide handoffs between coverage areas. In simulcast communication systems, a relatively simple handoff technique is used. Simulcast communication systems involve linking together the respective coverage areas of a plurality of communication sites to form a large wide area coverage area. The system typically employs communication channels which are common to each individual coverage area. As a mobile radio exits the coverage area of one site and enters the coverage area of another site, a conversation on the mobile radio is maintained because the linking of the multiple sites allows for simultaneous reception and broadcasting of the conversation at each site on the same channel.
Unlike simulcast communication systems, cellular communication systems do not employ common communication channels between the various sites. Rather, each coverage area employs a base site which includes a number of base stations for providing radiotelephones within the base site coverage area with a number of radiotelephone communication channels which are unique with respect to adjacent base site coverage areas. Each base site is controlled by the system's central switch controller.
A handoff between two base sites in a cellular communication system is primarily accomplished through communication between the radiotelephone and the radio equipment at the base site from which the radiotelephone is exiting. The base site equipment periodically measures the signal strength of the radiotelephone during the conversation, and, once it reaches a relatively low signal strength threshold, the same base site equipment sends a message to the adjacent base sites to determine which base site the radiotelephone is entering. The radiotelephone is then instructed to communicate on a selected channel from the base site equipment associated with the coverage area the radiotelephone is entering. Unfortunately, if the radiotelephone has already exited the coverage area of the base site sending the message or if the radiotelephone's reception of the message is momentarily inhibited by interference during the coverage area exit, the handoff will be unsuccessful, i.e., the conversation (call) will be dropped.
The frequency of this problem can be limited somewhat by increasing the overlap of the adjacent base site coverage areas. However, this reduces the overall coverage region of the system which can be compensated only by increasing the number of base sites and the cost.
Accordingly, a system for providing a handoff between coverage areas is needed which overcomes the aforementioned deficiencies.